Nonisothermal Aspects of Polymer Processing

Polypropylene is extruded at 200 °C from a film die having lips 76.2 cm wide and 0.1016 cm thick (see Fig. 5.1). The extruded film is drawn down to a width of 60.96 cm and a thickness of 0.005 cm. The distance from the die face to the casting drum is 2.54 cm. The film is in contact with the drum over a length of 70% of the circumference of the drum. The air temperature is taken as 25 °C and the line speed is 60 m/min. The radius of the drum is 0.45 m. Determine the heat transfer coefficient required at the drum surface to produce a clear film. The requirement for a clear film is based on keeping the crystallinity at the center to be less than 3% and the spherulite size less than 5000 ]xm. Tap water at 12 °C is available for cooling. 

Most polymer processes involve heat transfer. Polymers must usually be heated above their melting points before shaping and then cooled to maintain the desired shape. It is during the cooling phase of the process that the physical properties of the polymer can drastically be altered. Because the thermodynamic and thermal properties of most polymers are rather similar to other materials, it is not necessary to develop any new laws as it was for the flow of polymers. Hence, this chapter serves mostly as a review of heat transfer with emphasis on those topics pertinent to polymer processing. The main aspects that require additional discussion and that set polymers apart from other materials are their crystallization behavior and the ability to control molecular orientation during processing. 

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NONISOTHERMAL ASPECTS OF POLYMER PROCESSING we obtain the following differential equation ... 

The modern discipline of Materials Science and Engineering can be described as a search for experimental and theoretical relations between a material s processing, its resulting microstructure, and the properties arising from that microstructure. These relations are often complicated, and it is usually difficult to obtain closed-form solutions for them. For that reason, it is often attractive to supplement experimental work in this area with numerical simulations. During the past several years, we have developed a general finite element computer model which is able to capture the essential aspects of a variety of nonisothermal and reactive polymer processing operations. This "flow code" has been Implemented on a number of computer systems of various sizes, and a PC-compatible version is available on request. This paper is intended to outline the fundamentals which underlie this code, and to present some simple but illustrative examples of its use. 

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